Non-oriented electrical steel sheet and method for producing same, and motor core and method for producing same

ABSTRACT

A non-oriented electrical steel sheet with an average magnetostriction λp-p at 400 Hz and 1.0 T of not more than 4.5×10−6, and area ratio of recrystallized grains at a section in rolling direction of steel sheet of 40 to 95% and an average grain size of 10 to 40 μm is obtained by subjecting a steel slab containing, in mass %, C: not more than 0.005%, Si: 2.8 to 6.5%, Mn: 0.05 to 2.0%, Al: not more than 3.0%, P: not more than 0.20%, S: not more than 0.005%, N: not more than 0.005%, Ti: not more than 0.003%, V: not more than 0.005% and Nb: not more than 0.005% and satisfying Si—2Al—Mn≥0 to hot rolling, hot-band annealing, cold rolling and finish annealing under adequate cold rolling and finish annealing conditions, and a motor core is manufactured by such a steel sheet.

TECHNICAL FIELD

This invention relates to a non-oriented electrical steel sheet used as an iron core material for an automotive motor and a method for producing the same as well as a motor core using the above non-oriented electrical steel sheet and a method for producing the same.

BACKGROUND ART

With the worldwide increased demand for energy saving in electric instruments in recent years, non-oriented electrical steel sheets used for an iron core of a rotary machine are required to have more excellent magnetic properties. Moreover, there has been a strong need for smaller and more powerful drive motors for HEVs (Hybrid Electric Vehicles) and EVs (Electric Vehicles), and to meet this demand, the revolution number of the motor has been increased.

A motor core comprises a stator core and a rotor core. As the driving motor for HEV has a large outer diameter, a big centrifugal force is applied to the rotor core. Also, since the rotor core has a very narrow portion (1 to 2 mm) called as a rotor core bridge portion, depending on its configuration, a non-oriented electrical steel sheet used for a rotor core is desirable to have a strength higher than that of conventional ones. On the other hand, a non-oriented electrical steel sheet used for a stator core is desirable to be a higher magnetic flux density and a lower iron loss to attain smaller and more powerful motor. That is, the ideal property of the steel sheet used for the motor core is high strength for the rotor core while it is high magnetic flux density and low iron loss for the stator core.

As described above, the electrical steel sheet used for the same motor core is required to have largely different properties between the rotor core and the stator core. In the production of the motor core, however, it is desirable to take out a rotor core material and a stator core material at the same time from the same steel sheet material by blanking, and laminate each core material to assemble the rotor core or the stator core.

As a method for producing the non-oriented electrical steel sheet with a high strength and a low iron loss as mentioned above, for example, Patent Literature 1 discloses a technique for producing a high-strength rotor core and a low-iron loss stator core from the same steel material comprising producing a non-oriented electrical steel sheet having a high strength by subjecting a steel sheet after finish annealing to skin pass rolling of not less than 3% but less than 10% and taking out a rotor core material and stator core material from the steel sheet by blanking to assemble into a rotor core and a stator core, and thereafter subjecting only the stator core to a stress relief annealing. In this technique, impurities (Ti, S, N, V, Nb, Zr and As) contained in the steel are reduced to a very low level, and further Ni is added to promote crystal grain growth in the stress relief annealing.

CITATION LIST Patent Literature

Patent Literature 1: JP-A-2008-50686

SUMMARY OF INVENTION Technical Problem

In the technique disclosed in Patent Literature 1, however, the skin pass rolling is performed after the finish annealing, to attain high strength and promote grain growth in the stress relief annealing, which causes a problem that the production cost is increased. Furthermore, when Ni being high in the material cost is added in a substantial amount, there is a problem that the cost becomes higher.

The invention is made in consideration of the above problems inherent to the prior art, and an object thereof is to provide a non-oriented electrical steel sheet with a high strength and a low iron loss, which can be used as an iron core material for automotive motor, a motor core using the steel sheet, and a method for producing each product at a low cost.

Solution to Problem

First, the invention that has been developed to solve the above problems and achieve the above object provides

a non-oriented electrical steel sheet having a chemical composition comprising C: not more than 0.005 mass %, Si: 2.8 to 6.5 mass %, Mn: 0.05 to 2 mass %, Al: not more than 3.0 mass %, P: not more than 0.20 mass %, S: not more than 0.005 mass %, N: not more than 0.005 mass %, Ti: not more than 0.003 mass %, V: not more than 0.005 mass %, and Nb: not more than 0.005 mass %, provided that Si, Mn and Al satisfy Si—2Al—Mn≥0, and the remainder being Fe and inevitable impurities,

in which an average value of magnetostriction λ_(p-p) (L) in a rolling direction and magnetostriction λ_(p-p) (C) in a direction perpendicular to the rolling direction at a frequency of 400 Hz and a magnetic flux density of 1.0 T is not more than 4.5×10⁻⁶ and an area ratio of recrystallized crystal grains at a section in the rolling direction of the steel sheet is 40 to 95% and an average grain size is 10 to 40 μm.

The non-oriented electrical steel sheet according to the invention is preferable to contain at least one group selected from the following groups A to D, in addition to the above chemical composition:

Group A: 0.0020 to 0.10 mass % in total of one or two selected from Mo and W;

Group B: 0.005 to 0.20 mass % of one or two selected from Sn and Sb;

Group C: 0.001 to 0.010 mass % in total of one or two selected from Ca and Mg;

Group D: 0.01 to 1.0 mass % in total of one or more selected from Cu, Ni and Cr.

In the non-oriented electrical steel sheet according to the invention, it is preferable that a yield stress is not less than 500 MPa and a sheet thickness t (mm) and an iron loss W_(10/400) (W/kg) satisfy the following equation (1): W _(10/400)≤9+62×t  (1).

Secondly, the invention proposes a method for producing a non-oriented electrical steel sheet comprising a series of steps of

hot-rolling a steel slab having a chemical composition comprising C: not more than 0.005 mass %, Si: 2.8 to 6.5 mass %, Mn: 0.05 to 2.0 mass %, Al: not more than 3.0 mass %, P: not more than 0.20 mass %, S: not more than 0.005 mass %, N: not more than 0.005 mass %, Ti: not more than 0.003 mass %, V: not more than 0.005 mass %, and Nb: not more than 0.005 mass %, provided that Si, Mn and Al satisfy Si—2Al—Mn≥0, and the remainder being Fe and inevitable impurities to form a hot-rolled sheet,

subjecting the hot-rolled sheet to a hot-band annealing and then to a single cold rolling or two or more cold rollings having an intermediate annealing interposed therebetween to form a cold-rolled sheet having a final sheet thickness and

subjecting the cold-rolled sheet to a finish annealing,

in which an average rolling speed in a final cold rolling is 100 to 900 m/min;

a friction coefficient in a final pass is 0.01 to 0.10;

a ratio (R_(s)/R_(t)) of rolling reduction R_(s) in the final pass (%) to a total rolling reduction R_(t) (%) is 0.15 to 0.45; and

a soaking temperature in the finish annealing falls within the range of 700 to 820° C.

The steel slab used in the method for producing the non-oriented electrical steel sheet according to the invention is preferable to contain at least one group selected from the following groups A to D in addition to the above chemical composition:

Group A: 0.0020 to 0.10 mass % in total of one or two selected from Mo and W;

Group B: 0.005 to 0.20 mass % of one or two selected from Sn and Sb;

Group C: 0.001 to 0.010 mass % in total of one or two selected from Ca and Mg;

Group D: 0.01 to 1.0 mass % in total of one or more selected from Cu, Ni and Cr.

Thirdly, the invention provides a motor core comprising a stator core and a rotor core formed by laminating any one of the above-described non-oriented electrical steel sheets, in which an average crystal grain size of the rotor core is 10 to 40 μm and an average crystal grain size of the stator core is not less than 80 μm.

Fourthly, the invention proposes a method for producing the above motor core, in which the stator core formed by laminating the non-oriented electrical steel sheet having an average crystal grain size of 10 to 40 μm is subjected to a stress relief annealing thereby to have the average crystal grain size of not less than 80 μm.

Advantageous Effects of Invention

According to the invention, the rotor core required to have a high strength and the stator core required to have a low iron loss can be produced from the same non-oriented electrical steel sheet. Thus, it is possible to stably provide an iron core material for an automotive motor at a low cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing a relation between magnetostriction λ_(p-p) and iron loss W_(10/400) of a steel sheet after a finish annealing.

DESCRIPTION OF EMBODIMENTS

First, there will be described an example of experiments that have led to the development of the invention.

<Experiment 1>

In order to examine an influence of magnetostriction on an iron loss W_(10/400) in a high frequency band, steel slab A to H having a chemical composition shown in Table 1 are each hot rolled to form a hot-rolled sheet with a sheet thickness of 2.0 mm, and then the hot-rolled sheet is subjected to a hot-band annealing at 960° C. for 30 seconds, pickled and cold-rolled once to form a cold-rolled sheet with a final sheet thickness of 0.30 mm (total rolling reduction R_(t): 85%). The cold-rolled sheet is then subjected to a finish annealing in a non-oxidizing atmosphere of 20 vol % H₂—80 vol % N₂ at 750° C. for 10 seconds to form a non-oriented electrical steel sheet.

Here, the cold rolling to obtain the final sheet thickness is conducted with a 5-stand tandem rolling mill under conditions that an average rolling speed in each stand is 600 m/min and a friction coefficient in the final stand is 0.03 and the rolling reduction R_(s) in the final stand is 25% (R_(s)/R_(t)=0.29).

Thereafter, samples in L-direction and C-direction of a width of 30 mm and a length of 280 mm are taken out from the steel sheet after the finish annealing, and a magnetostriction λ_(p-p) of the steel sheet after the finish annealing (frequency: 400 Hz, magnetic flux density: 1.0 T) is measured using a laser displacement gauge from the samples.

Next, an iron loss W_(10/400) of each sample after the measurement of the magnetostriction is measured by an Epstein test in accordance with JIS C2550-1:2011.

Further, a JIS No. 5 tension specimen is taken out from the sample in L-direction after the measurement of the magnetostriction and subjected to a tensile test conducted in accordance with JIS Z2241:2011 to measure a yield stress (upper yield point).

Moreover, a specimen of 15 mm in L-direction and 10 mm in C-direction is taken out from the sample, and the sectional face in the rolling direction of the steel sheet (section perpendicular to the widthwise direction of the sheet) is polished, etched and then observed by an optical microscope to measure a recrystallization ratio (area ratio of recrystallized grains) and an average grain size of the recrystallized grains. The average grain size of the recrystallized grains means a diameter equivalent to a circle that is obtained by shooting a sectional area of the steel sheet (sheet thickness×1000 μm) from 3 viewing fields, measuring individual areas of the recrystallized crystal grains through image processing of the shot images and calculating an average value from the measured areas.

The measurement results are also shown in Table 1. As seen from these results, all the steel sheets after the finish annealing in this experiment have a recrystallization ratio of 75% and a microstructure where a non-recrystallization structure with an average grain size of the recrystallized crystal grains of 20 μm remains, and a yield stress of not less than 500 MPa is obtained. In the steel sheets where the value of Si—2Al—Mn is not more than 0, the magnetostriction λ_(p-p) and iron loss W_(10/400) indicate big values.

TABLE 1 Steel Chemical composition(mass %) symbol C P Si Mn Al S N O Ti V Nb Si—2Al—Mn A 0.0022 0.01 3.3 0.4 0.8 0.0015 0.0021 0.0021 0.0011 0.0008 0.0011 1.3 B 0.0018 0.01 3.8 0.5 0.7 0.0013 0.0019 0.0018 0.0014 0.0012 0.0013 1.9 C 0.0017 0.01 3.5 0.9 1.2 0.0018 0.0018 0.0017 0.0015 0.0011 0.0014 0.2 D 0.0028 0.01 2.5 1.2 1.2 0.0019 0.0017 0.0024 0.0014 0.0012 0.0012 −1.1 E 0.0024 0.01 2.8 1.6 1.5 0.0014 0.0023 0.0023 0.0012 0.0011 0.0009 −1.8 F 0.0027 0.01 3.0 1.3 1.6 0.0018 0.0018 0.0029 0.0011 0.0009 0.0011 −1.5 G 0.0018 0.01 4.1 0.2 0.5 0.0015 0.0019 0.0014 0.0011 0.0012 0.0008 2.9 H 0.0023 0.01 3.9 0.4 0.7 0.0017 0.0017 0.0021 0.0010 0.0011 0.0012 2.1 Steel sheet properties after finish annealing Average magneto- Iron Average striction loss Area ratio of grain size of Yield Steel value λ_(p−p) W_(10/400) recrystallized recrystallized stress symbol (×10⁻⁶) (W/kg) grains (%) grains (μm) (MPa) Remarks A 2.4 21 75 20 550 Invention Example B 1.6 19 75 20 608 Invention Example C 4.1 24 75 20 615 Invention Example D 5.2 32 75 20 505 Comparative Example E 8.2 40 75 20 561 Comparative Example F 7.2 38 75 20 586 Comparative Example G 1.3 19 75 20 625 Invention Example H 1.8 19.5 75 20 618 Invention Example

FIG. 1 also shows a relation between magnetostriction λ_(p-p) (average value of the values in L-direction and C-direction) and iron loss W_(10/400) after the finish annealing. As seen from this FIGURE, the magnetostriction λ_(p-p) after the finish annealing is interrelated with the iron loss W_(10/400), and there is a tendency that as the magnetostriction λ_(p-p) becomes smaller, the iron loss W_(10/400) decreases, and further when the magnetostriction λ_(p-p) is not more than 4.5×10⁻⁶, an excellent iron loss property is obtained with the iron loss W_(10/400) of not more than 27.6 W/kg. This is considered due to the fact that as the magnetostriction increases, the magnetoelastic energy increases and the hysteresis loss largely increases.

<Experiment 2>

In order to further reduce the iron loss and increase the strength, the following experiment is conducted based on the above results.

A steel slab having a chemical composition comprising C: 0.0027 mass %, Si: 3.61 mass %, Mn: 0.31 mass %, P: 0.01 mass %, S: 0.0018 mass %, Al: 1.2 mass %, N: 0.0017 mass %, O: 0.0023 mass %, Ti: 0.0013 mass %, Nb: 0.0007 mass %, V: 0.0008 mass % and the remainder being Fe and inevitable impurities is hot rolled to form a hot-rolled sheet having a sheet thickness of 1.9 mm. The hot-rolled sheet is subjected to a hot-band annealing at 940° C. for 30 seconds, pickled, and cold rolled once to obtain a cold-rolled sheet having a final sheet thickness of 0.25 mm (total rolling reduction R_(t)=87%). The cold-rolled sheet is then subjected to a finish annealing in a non-oxidizing atmosphere of 20 vol % H₂—80 vol % N₂ at 790° C. for 10 seconds to obtain a non-oriented electrical steel sheet.

Thereafter, samples in L-direction and C-direction of a width of 30 mm and a length of 280 mm are taken out from a widthwise central portion of the steel sheet after the finish annealing (every 100 m in the coil), and an iron loss W_(10/400) of the steel sheet after the finish annealing is measured in accordance with JIS C2550-1:2011.

Moreover, a JIS No. 5 tension specimen is taken out from the sample in L-direction after the measurement of the iron loss and subjected to a tensile test thereof in accordance with according to JIS Z2241:2011 to measure a yield stress YS (upper yield point).

The measurement result shows that there is large variation both in the iron loss and the yield stress. As a result of an examination which is made to examine the cause of the variation, it is considered that the conditions during the cold rolling (rolling speed, friction coefficient, and rolling reduction distribution) may have influenced the results.

In order to examine the influence of the conditions during the cold rolling upon the magnetostriction and iron loss after the finish annealing, a hot-rolled sheet having the same chemical composition as in the above steel slab and after hot-band annealing is used as a raw material and subjected to a final cold rolling with a 5-stand tandem rolling machine by variously changing an average rolling speed, a friction coefficient in the final pass and a ratio (R_(s)/R_(t)) of rolling reduction R_(s) in final pass to total rolling reduction R_(t) to form a cold-rolled sheet having a final sheet thickness of 0.25 mm. Thereafter, the cold-rolled sheet is subjected to a finish annealing in a non-oxidizing atmosphere of 20 vol % H₂—80 vol % N₂ at 790° C. for 10 seconds to obtain a non-oriented electrical steel sheet. Here, the friction coefficient in the final pass of the final cold rolling is adjusted by changing the rolling oil.

Then, samples are taken out from a position corresponding to each rolling condition of the steel sheet (coil) after the finish annealing, and the magnetostriction λ_(p-p), iron loss W_(10/400) and yield stress are measured as in Experiment 1, and the area ratio (recrystallization ratio) and average crystal grain size of recrystallized grains observed at a section in the rolling direction are also measured.

TABLE 2 Rolling conditions of final cold rolling Rolling Steel sheet properties after finish annealing Average Friction Total reduction Average rolling coefficient rolling Rs of Magnetostriction Iron loss Area ratio of grain size of Yield Steel speed in final reduction final pass average value W_(10/400) recrystallized recrystallized stress No. (m/min) pass R_(t) (%) (%) R_(s)/R_(t) λ_(p−p)(×10⁻⁶) (W/kg) grains (%) grains (μm) (MPa) Remarks 1 50 0.02 86.8 21 0.24 8.1 35.0 35 25 570 Comparative Example 2 200 0.02 86.8 21 0.24 3.5 23.2 45 28 565 Invention Example 3 400 0.02 86.8 21 0.24 2.9 22.1 55 29 563 Invention Example 4 600 0.02 86.8 21 0.24 2.7 21.0 70 32 558 Invention Example 5 800 0.02 86.8 21 0.24 2.5 20.2 85 35 550 Invention Example 6 1000 0.02 86.8 21 0.24 2.1 19.5 98 55 480 Comparative Example 7 650 0.005 86.8 28 0.32 7.6 34.2 30 20 560 Comparative Example 8 650 0.02 86.8 28 0.32 2.5 20.0 60 24 560 Invention Example 9 650 0.08 86.8 28 0.32 2.9 19.3 70 28 554 Invention Example 10 650 0.15 86.8 28 0.32 3.9 20.5 80 50 475 Comparative Example 11 650 0.20 86.8 28 0.32 3.7 20.4 98 55 465 Comparative Example 12 500 0.03 86.8 10 0.12 7.5 36.0 30 22 570 Comparative Example 13 500 0.03 86.8 20 0.23 2.5 19.5 55 25 555 Invention Example 14 500 0.03 86.8 30 0.35 2.6 19.6 70 28 540 Invention Example 15 500 0.03 86.8 45 0.52 3.6 21.1 98 50 470 Comparative Example 16 500 0.03 86.8 50 0.58 3.4 20.5 98 55 460 Comparative Example

The measurement results are also shown in Table 2. As seen from these results, as the average rolling speed is made lower, the friction coefficient in the final pass is made lower and the rolling reduction ratio in the final pass is decreased, the area ratio of the recrystallized grains decreases and the average grain size of the recrystallized grains becomes smaller and and the yield stress increases. However, when the average rolling speed is made too low, the friction coefficient in the final pass is made too low, or the rolling reduction in the final pass is extremely decreased, the magnetostriction increases rapidly and the iron loss largely increases.

From the above result, it is confirmed that the cause of the variation in the iron loss property and strength in the coil is largely influenced by the variation of the final cold rolling conditions in the coil.

The mechanism how the cold rolling conditions influence the magnetostriction properties and the area ratio and average grain size of recrystallized grains after finishing annealing is not sufficiently clear at present, but it is considered that the strain amount to be introduced and the deformation mechanism are changed by the variation of the cold rolling conditions, whereby the recrystallization behavior and the grain growth behavior in the subsequent finish annealing are changed to change the recrystallization ratio and recrystallization texture after the finish annealing, which affects the magnetostriction property and strength.

The invention is developed by further studying the above novel knowledge.

There will be described the properties of the non-oriented electrical steel sheet according to the invention below.

Yield stress: not less than 500 MPa

A rotor core is generally produced by shaping a steel sheet after finish annealing into a core by blanking or the like, laminating, and clamping (fixing) by welding, caulking or the like, and is never subjected to a stress relief annealing. As previously described, a large centrifugal force is applied to the rotor core. Therefore, the steel sheet used for the rotor core is desirable to be high in the strength after the finish annealing also as described above. Further, the fatigue strength (fatigue limit) of the steel sheet usually becomes higher as the yield stress becomes higher. In the invention, therefore, the preferable yield stress of the steel sheet after the finish annealing is defined to be not less than 500 MPa. More preferably, it is not less than 520 MPa. Here, the yield stress is an upper yield point when a JIS No. 5 tension specimen is subjected to a tensile test in accordance with JIS Z2241:2011.

Area ratio of recrystallized grains: 40 to 95%, Average grain size of recrystallized grains: 10 to 40 μm

In the non-oriented electrical steel sheet according to the invention, the yield stress after the finish annealing is desirable to be not less than 500 MPa as described above. In order to ensure the above strength in the non-oriented electrical steel sheet according to the invention, it is necessary that the area ratio of the recrystallized grains (recrystallization ratio) after the finish annealing is 40 to 95% and the average grain size of the recrystallized grains is 10 to 40 μm. When the area ratio of the recrystallized grains is less than 40% and the average grain size of the recrystallized grains is less than 10 μm, the recrystallization is insufficient and the magnetostriction becomes larger and the iron loss largely increases, as previously described. On the other hand, when the area ratio of the recrystallized grains exceeds 95% and the average grain size of the recrystallized grains exceeds 40 μm, the recrystallization is too promoted on the contrary to ensure the yield stress of not less than 500 MPa. Preferably, the area ratio of the recrystallized grains is in the range of 50 to 90%, and the average grain size of the recrystallized grains is in the range of 15 to 30 μm.

Iron loss W_(10/400): not more than 9+62t (W/kg) (t: sheet thickness (mm))

The rotor core of the motor is preferable have a low iron loss, because heat generation causes a problem in addition to the strength. In the invention, therefore, an iron loss W_(10/400) (frequency: 400 Hz, magnetic flux density B=1.0T) (W/kg) is used as an indicator of the iron loss property in accordance with the driving/controlling conditions of HEV driving motor, and the iron loss W_(10/400) is defined to satisfy the following equation (1) in a relation to a product sheet thickness (final cold rolled sheet thickness) t (mm): W _(10/400)≤9+62×t  (1). This is due to the fact that the iron loss value is dependent on the sheet thickness and, the heat generation of the motor core becomes larger and the motor efficiency considerably lowers when the iron loss value does not satisfy the above equation (1).

Magnetostriction λ_(p-p): not more than 4.5×10⁻⁶

As shown in FIG. 1 , the iron loss W_(10/400) of the steel sheet after the finish annealing is strongly interrelated with a magnetostriction λ_(p-p), and hence the value of the iron loss W_(10/400) after the finish annealing can be reduced by decreasing the magnetostriction λ_(p-p) after the finish annealing. In the invention, therefore, the magnetostriction λ_(p-p) after the finish annealing is limited to not more than 4.5×10⁻⁶ where the iron loss W_(10/400) satisfies the equation (1) in FIG. 1 . Preferably, it is not more than 4.0×10⁻⁶. Moreover, the value of the magnetostriction λ_(p-p) is an average value of magnetostrictions λ_(p-p) in the rolling direction (L) and the direction (C) perpendicular to the rolling direction measured at a frequency of 400 Hz and a magnetic flux density B of 1.0 T.

There will be described a chemical composition of a raw steel material (steel slab) used in a non-oriented electrical steel sheet according to the invention and a product sheet.

C: not more than 0.0050 mass %

C contained in the product sheet is a harmful element that forms a carbide to cause magnetic aging and deteriorates the iron loss property. Therefore, C contained in the raw material is limited to 0.0050 mass % as an upper limit. It is preferably not more than 0.0040 mass %. The lower limit of C is not particularly defined, but is preferable to be about 0.0001 mass % from a viewpoint of suppressing decarburization cost in the refining step.

Si: 2.8 to 6.5 mass %

Si has an effect of increasing a specific resistance of steel to reduce the iron loss and increasing a strength of steel by solid-solution strengthening. Therefore, Si is contained in an amount of not less than 2.8 mass %. On the other hand, when it exceeds 6.5 mass %, it becomes difficult to perform the rolling, so that the upper limit is 6.5 mass %. Preferably, Si falls within the range of 3.0 to 6.0 mass %.

Mn: 0.05 to 2.0 mass %

Mn is an element useful for increasing the specific resistance and strength of steel, like Si, and forms a sulfide to suppress hot shortness. Therefore, Mn is contained in an amount of not less than 0.05 mass %. On the other hand, the addition of Mn exceeding 2.0 mass % causes slab cracking or the like and deteriorates operability in the steel-making step, so that the upper limit is 2.0 mass %. Preferably, it falls within the range of 0.1 to 1.5 mass %.

P: not more than 0.20 mass %

P is an element useful for adjusting the strength (hardness) of steel. However, when it exceeds 0.20 mass %, the steel causes embrittlement and it becomes difficult to perform the rolling, and therefore the upper limit is 0.20 mass %. The lower limit is not particularly defined, but it is preferable to be about 0.001 mass % from a viewpoint of suppressing the dephosphorization cost in the refining step. Preferably, P falls within the range of 0.01 to 0.1 mass %.

Al: not more than 3.0 mass %

Al is a useful element having an effect of increasing the specific resistance of steel and reducing the iron loss, like Si. However, when it exceeds 3.0 mass %, it is difficult to perform the rolling, so that the upper limit of Al is 3.0 mass %. Preferably, it is not more than 2.0 mass %.

Moreover, when the Al content is in the range of more than 0.01 mass % to less than 0.10 mass %, fine MN is precipitated to increase the iron loss, and hence Al content is preferable to be not more than 0.01 mass % or not less than 0.10 mass %. Especially, when Al is decreased, the texture is improved and the magnetic flux density increases. Therefore, when the magnetic flux density is considered important, Al is preferable to be not more than 0.01 mass %. More preferably, it is not more than 0.003 mass %.

S, N, Nb and V: not more than 0.005 mass %

S, N, Nb and V are elements that form fine precipitates, and block grain growth in stress relief annealing thereby to badly affect the iron loss property. In particular, when each element contents exceeds 0.005 mass %, the bad influence becomes remarkable. Therefore, the upper limit of each element is limited to 0.005 mass %. More preferably, it is not more than 0.003 mass %.

Ti: not more than 0.003 mass %

Ti is an element that likewise forms fine precipitates and blocks the grain growth in stress relief annealing to badly affect the iron loss property. Especially, when Ti content exceeds 0.003 mass %, the bad influence becomes remarkable. Therefore, the upper limit is limited to 0.003 mass %. Preferably, the upper limit is not more than 0.002 mass %. Si—2Al—Mn≥0

In the non-oriented electrical steel sheet according to the invention, the above ingredients satisfy the composition of the above given ranges, and moreover, the contents of Si, Al and Mn (mass %) are necessary to satisfy the following equation (2): Si—2Al—Mn≥0  (2).

When the above contents are out of the equation (2), i.e., the left side value of the equation (2) is less than 0, hysteresis loss after the finish annealing at a frequency of 400 Hz and a magnetic flux density B of 1.0 T increases and the magnetostriction λ_(p-p) also increases. Moreover, the left side value of the equation (2) is preferably not less than 0.2 mass %.

Moreover, the non-oriented electrical steel sheet according to the invention may contain the following ingredients in addition to the above ingredients.

One or two selected from Mo and W: 0.0020 to 0.10 mass % in total

Mo and W are elements effective for preventing surface defects (scab) from occurring in the steel sheet. In particular, the non-oriented electrical steel sheet according to the invention is a high alloy steel, which is apt to be oxidized on its surface, and thus scabs may be caused due to surface cracking. However, the above cracking can be suppressed by adding Mo and/or W being an element increasing a high-temperature strength. This effect is insufficient when the total content of Mo and W is less than 0.0020 mass %, while when the total content exceeds 0.10 mass %, the above effect is saturated, only resulting in increase in the alloy cost. Therefore, when Mo and/or W is added, the total content of Mo and W is preferable to be in the range of 0.0020 to 0.10 mass %. More preferably, the total content is in a range of 0.0050 to 0.050 mass %.

One or two selected from Sn and Sb: 0.005 to 0.20 mass % each

Sn and Sb have an effect of improving the recrystallization texture and improving the magnetic flux density and iron loss property. In order to obtain such an effect, it is necessary to add each element in an amount of not less than 0.005 mass %. However, when the addition amount exceeds 0.20 mass %, the above effect is saturated. Therefore, when Sn and/or Sb is added, each content is preferable to be in the range of 0.005 to 0.20 mass %, more preferably, 0.01 to 0.1 mass %.

One or two selected from Ca and Mg: 0.001 to 0.010 mass % in total

Ca and Mg each have an effect of forming stable sulfide or selenide to improve the grain growth in the stress relief annealing. In order to obtain this effect, it is necessary to add Ca and/or Mg in a total amount of not less than 0.001 mass %, while when the addition amount exceeds 0.010 mass %, the iron loss rather increases. Therefore, when Ca and/or Mg are added, the total amount is preferable to fall within the range of 0.001 to 0.010 mass %. More preferably, the total amount of Ca and Mg falls within the range of 0.003 to 0.008 mass %.

One or more selected from Cu, Ni and Cr: 0.01 to 1.0 mass % in total

Cu, Ni and Cr have an effect of increasing the specific resistance of steel, reducing the iron loss and increasing the strength of steel. To obtain the above effect, it is necessary to add one or more selected from Cu, Ni and Cr in a total amount of not less than 0.01 mass %. However, the addition of not less than 1.0 mass % brings about the increase in the cost. Therefore, the total addition amount of the above elements is preferable to fall within the range of 0.01 to 1.0 mass %, more preferably 0.1 to 0.5 mass %. Further preferably, it is not less than 0.1 mass % and less than 0.5 mass %.

Next, the method for producing a non-oriented electrical steel sheet according to the invention will be described below.

The method for producing a non-oriented electrical steel sheet according to the invention comprises a series of steps of

producing a raw steel material (slab) having the above chemical composition,

hot rolling the slab to form a hot-rolled sheet,

subjecting the hot-rolled sheet to a hot-band annealing and a single or two or more cold rollings having an intermediate annealing interposed therebetween to obtain a cold-rolled sheet having a final sheet thickness and

subjecting the cold-rolled sheet to a finish annealing. This method will be concretely described below.

At first, the steel slab used in the production of a non-oriented electrical steel sheet according to the invention can be produced by melting a steel having a chemical composition adapted to the invention by a known refining process using a converter, an electric furnace, a vacuum degassing device or the like and then conducting a usual continuous casting method or an ingot making—blooming method. Moreover, a thin cast slab having a thickness of not more than 100 mm may be produced by a direct casting method.

Then, the steel slab is hot rolled by a known method to form a hot-rolled sheet. The steel slab is usually subjected to the hot rolling after reheating to a given temperature in a heating furnace, but may be subjected to the hot rolling immediately after casting without reheating. When the thin cast slab is used, it may be subjected to hot rolling or may be put forward to the subsequent step without hot rolling.

In the hot-band annealing subsequent to the hot rolling, the soaking temperature is preferably in the range of 800 to 1100° C. When it is lower than 800° C., the effect of the hot-band annealing is small and the effect of improving the magnetic properties cannot be obtained sufficiently, while when it exceeds 1100° C., the crystal grains may be coarsened to promote brittle fracture (sheet breakage) in the cold rolling or it may become disadvantageous in the production cost. Moreover, a soaking time is preferable to be not more than 3 minutes from a viewpoint of ensuring the productivity. More preferably, the soaking temperature is 850 to 1000° C., and the soaking time is not more than 1 minute.

Thereafter, the hot-rolled sheet after the hot-band annealing is pickled and subjected to a single cold rolling or two or more cold rollings having an intermediate annealing interposed therebetween to obtain a cold-rolled sheet with a final sheet thickness. In this case, it is important to adjust an average rolling speed in the final cold rolling to obtain the final sheet thickness, a friction coefficient in the final pass and a ratio (R_(s)/R_(t)) of rolling reduction in the final pass to total rolling reduction to respective given ranges that will described later, from a viewpoint of achieving both the iron loss and strength, as previously described. Moreover, the final sheet thickness (product sheet thickness) is preferable to be in the range of 0.1 to 0.35 mm. The productivity decreases when the final sheet thickness exceeds 0.35 mm, while the effect of reducing the iron loss is small when it is less than 0.1 mm.

The average rolling speed in the final cold rolling is necessary to be in the range of 100 to 900 m/min. As previously described, when the average rolling speed in the final cold rolling is less than 100 m/min, the magnetostriction increases and the iron loss largely increases. While when the average rolling speed exceeds 900 m/min, the crystal grain size of the recrystallized grains becomes larger and the strength decreases. The preferable average rolling speed falls within the range of 200 to 800 m/min.

Here, the average rolling speed means the average rolling speed of 1 to n-passes when the final cold rolling is performed at n passes using a Sendzimir mill, and the average rolling speed of 1 to n-stands when a n-stand tandem rolling machine is used.

The friction coefficient of the final pass in the final cold rolling is necessary to fall within the range of 0.01 to 0.10. As previously described, when the friction coefficient of the final pass is less than 0.01, the magnetostriction increases and the iron loss largely increases. When it exceeds 0.10, the average grain size of the recrystallized grains becomes larger and the strength is decreased. Preferably, the friction coefficient of the final pass falls within the range of 0.02 to 0.08.

The friction coefficient can be determined from the sheet thickness, tension, rolling load, deformation resistance and the like by a Bland & Ford formula. Also, the friction coefficient can be adjusted by changing the rolling oil, adjusting roughness in the rolling roll or the like.

Further, the ratio (R_(s)/R_(t)) of rolling reduction R_(s) in the final pass to total rolling reduction R_(t) in the final cold rolling is necessary to be controlled to in the range of 0.15 to 0.45. As previously described, when R_(s)/R_(t) is less than 0.15, the magnetostriction becomes larger and the iron loss also increases. When R_(s)/R_(t) exceeds 0.45, on the other hand, the average grain size of the recrystallized grains increases to decrease the strength. The preferable R_(s)/R_(t) is in the range of 0.20 to 0.40.

Next, the cold-rolled sheet having the final sheet thickness is subjected to a finish annealing. The finish annealing is preferably a continuous annealing at an annealing temperature (soaking temperature) of 700 to 820° C. When the soaking temperature is lower than 700° C., the recrystallization is insufficient, so that the area ratio of the recrystallized grains of not less than 40% and the average grain size of not less than 10 μm cannot be attained and the good magnetic properties cannot be obtained. In addition to that, the effect of correcting the shape in the continuous annealing is not obtained sufficiently. When the soaking temperature exceeds 820° C., on the other hand, the recrystallization is rather promoted excessively to cause the area ratio of the recrystallized grains to exceed 95% and coarsen the crystal grain size to be more than 40 μm, resulting that the yield stress of not less than 500 MPa cannot be ensured after the finish annealing. Also, the soaking time is preferable to be in the range of 1 to 300 seconds. Moreover, the finish annealing conditions are lower temperature and shorter time as much as possible within a range capable of correcting the shape from a viewpoint of ensuring the strength after the finish annealing required in the rotor core. Concretely, the conditions are preferable to be on the range of 720 to 800° C. and 1 to 20 seconds.

Thereafter, it is preferable to apply an insulation coating to the surface of the steel sheet after the finish annealing to obtain the insulation property in the lamination. An organic coating containing a resin is preferable to be selected as the insulation coating in order to obtain a good blanking property, while a semi-organic or inorganic coating is preferable to be selected when the weldability is considered important.

When a material having an area ratio of recrystallized grains of not more than 95% and a crystal grain size of not more than 40 μm is used for the stator core of the motor, there is caused a problem that the iron loss is increased and the motor efficiency is lowered. Against such a problem, for example, when the iron loss ratio of the rotor core is small and the iron loss of the stator core is dominant with respect to the iron loss of the entire motor as in a core of permanent magnet type motor such as SPM, IPM or the like used in a power source of a hybrid vehicle (HEV), it is effective, in the rotor core, to ensure the strength with the recrystallization ratio and crystal grain size kept small, while, in the stator core, it is effective to perform a stress relief annealing after assembly of the core to increase the crystal grain size and reduce the iron loss. In order to obtain such an effect of reducing the iron loss, it is preferable that the area ratio of the recrystallized grains is 100% and the average grain size is not less than 80 μm after the stress relief annealing. The average grain size is more preferably not less than 90 μm, further preferably not less than 100 μm. Moreover, the stress relief annealing can be conducted under usual conditions, that for example, a soaking temperature is 700 to 950° C. and a soaking time is 10 to 300 minutes.

EXAMPLES

Steel slabs A to BC having different chemical compositions shown in Tables 3-1 and 3-2 are each heated at 1100° C. for 30 minutes and hot rolled to form a hot-rolled sheet having a sheet thickness of 2.0 mm. The hot-rolled sheet is subjected to a hot-band annealing at 980° C. for 30 seconds, cold rolled once, and rolled under conditions shown in Tables 4-1 and 4-2 using a 4 stand tandem rolling machine to obtain cold-rolled sheets having various final sheet thicknesses. Thereafter, each cold rolled sheet is subjected to a finish annealing by soaking at a temperature shown in Tables 4-1 and 4-2 for 10 seconds to produce a non-oriented electrical steel sheet (product sheet).

Then, samples in L-direction and C-direction of a width of 30 mm and a length of 280 mm are taken out from the central portion in the widthwise direction of the product sheet to measure an average magnetostriction λ_(p-p) in L-direction and C-direction using a laser displacement gauge and an iron loss W_(10/400) by an Epstein test. Also, JIS No. 5 tension specimen is taken out from the L-direction sample after the measurement of the magnetostriction and iron loss, and a tensile test thereof is conducted according to JIS Z2241:2011 to measure a yield stress (upper yield point). Further, a sample of 15 mm×10 mm is taken out from the above L-direction sample, and a section texture in the rolling direction thereof is observed to measure an area ratio (recrystallization ratio) of the recrystallized grains after the finish annealing and an average grain size of the recrystallized grains.

The measurement results are also shown in Tables 4-1 and 4-2. As seen from these results, all of the steel sheets after the finish annealing produced from the raw steel material having the chemical composition adapted to the invention under the conditions adapted to the invention have a yield stress of not less than 500 MPa and are excellent in the iron loss property.

TABLE 3-1 Steel Chemical composition (mass %) symbol C P Si Mn Al S N O Ti Nb A 0.0022 0.01 3.5 0.40 0.90 0.0015 0.0024 0.0017 0.0013 0.0016 B 0.0018 0.01 3.7 0.40 0.60 0.0019 0.0024 0.0014 0.0009 0.0014 C 0.0019 0.01 1.8 0.40 0.80 0.0014 0.0024 0.0017 0.0013 0.0016 D 0.0019 0.01 4.5 0.20 0.30 0.0017 0.0021 0.0019 0.0011 0.0012 E 0.0021 0.01 6.8 0.50 0.30 0.0017 0.0021 0.0019 0.0011 0.0012 F 0.0024 0.01 3.4 1.20 0.30 0.0019 0.0029 0.0024 0.0013 0.0016 G 0.0024 0.01 3.4 2.50 0.80 0.0019 0.0029 0.0024 0.0013 0.0016 H 0.0022 0.01 3.8 0.50 0.001 0.0019 0.0031 0.0028 0.0012 0.0011 I 0.0022 0.01 3.8 0.50 0.001 0.0019 0.0031 0.0028 0.0012 0.0011 J 0.0022 0.01 3.8 0.50 0.001 0.0019 0.0031 0.0028 0.0012 0.0008 K 0.0022 0.08 3.8 0.50 0.001 0.0019 0.0031 0.0028 0.0004 0.0011 L 0.0022 0.24 3.8 0.50 0.001 0.0019 0.0024 0.0023 0.0012 0.0011 M 0.0022 0.01 3.8 0.50 0.001 0.0019 0.0031 0.0028 0.0012 0.0011 N 0.0022 0.01 3.0 0.50 0.001 0.0019 0.0021 0.0018 0.0008 0.0006 O 0.0019 0.01 3.4 0.20 1.40 0.0014 0.0024 0.0017 0.0013 0.0016 P 0.0019 0.01 3.35 0.20 1.50 0.0014 0.0024 0.0017 0.0013 0.0016 Q 0.0019 0.01 2.9 0.40 3.60 0.0014 0.0024 0.0017 0.0013 0.0016 R 0.0022 0.01 3.5 0.50 0.001 0.0019 0.0059 0.0028 0.0012 0.0011 S 0.0022 0.01 3.5 0.50 0.001 0.0019 0.0018 0.0064 0.0012 0.0011 T 0.0022 0.01 3.5 0.50 0.001 0.0019 0.0018 0.0021 0.0045 0.0011 U 0.0022 0.01 3.5 0.50 0.001 0.0019 0.0018 0.0023 0.0012 0.0057 V 0.0022 0.01 3.5 0.50 0.001 0.0019 0.0018 0.0023 0.0012 0.0014 W 0.0024 0.01 3.6 0.60 0.65 0.0018 0.0022 0.0025 0.0011 0.0011 X 0.0019 0.01 3.4 0.60 0.65 0.0021 0.0024 0.0024 0.0010 0.0010 Y 0.0027 0.01 3.7 0.50 0.65 0.0017 0.0023 0.0021 0.0009 0.0009 Z 0.0015 0.01 3.5 0.40 0.65 0.0018 0.0021 0.0024 0.0012 0.0013 Steel Chemical composition (mass %) symbol V Sn Sb Others Si—2Al—Mn Remarks A 0.0011 0.001 0.001 — 1.3 Invention steel B 0.0007 0.04 0.001 — 2.1 Invention steel C 0.0011 0.001 0.001 — −0.2 Comparative steel D 0.0009 0.001 0.001 — 3.7 Invention steel E 0.0009 0.001 0.001 — 5.7 Comparative steel F 0.0011 0.001 0.001 — 1.6 Invention steel G 0.0011 0.001 0.001 — −0.7 Comparative steel H 0.0008 0.001 0.001 — 3.3 Invention steel I 0.0008 0.10 0.001 — 3.3 Invention steel J 0.0008 0.001 0.05 — 3.3 Invention steel K 0.0008 0.001 0.001 — 3.3 Comparative steel L 0.0008 0.001 0.001 — 3.3 Invention steel M 0.0008 0.001 0.001 Ca: 0.003 3.3 Invention steel N 0.0008 0.001 0.001 Mg: 0.002 2.5 Invention steel O 0.0011 0.001 0.001 — 0.4 Invention steel P 0.0011 0.001 0.001 — 0.2 Invention steel Q 0.0011 0.001 0.001 — −4.7 Comparative steel R 0.0008 0.001 0.001 — 3.0 Comparative steel S 0.0008 0.001 0.001 — 3.0 Comparative steel T 0.0008 0.001 0.001 — 3.0 Comparative steel U 0.0008 0.001 0.001 — 3.0 Comparative steel V 0.0062 0.001 0.001 — 3.0 Comparative steel W 0.0008 0.001 0.001 W: 0.0027 1.7 Invention steel X 0.0007 0.001 0.001 W: 0.0058 1.5 Invention steel Y 0.0014 0.001 0.001 Mo: 0.0035 1.9 Invention steel Z 0.0012 0.001 0.001 W: 0.0049, Mo: 0.0028 1.8 Invention steel

TABLE 3-2 Steel Chemical Composition (mass %) symbol C P Si Mn Al S N O Ti Nb AA 0.0021 0.01 3.6 0.50 0.60 0.0019 0.0020 0.0024 0.0013 0.0013 AB 0.0023 0.01 3.5 0.60 0.90 0.0016 0.0021 0.0024 0.0012 0.0013 AC 0.0021 0.01 3.5 0.60 0.90 0.0018 0.0024 0.0021 0.0012 0.0011 AD 0.0024 0.01 3.5 0.60 0.90 0.0018 0.0021 0.0024 0.0012 0.0013 AE 0.0025 0.01 3.5 0.60 0.90 0.0017 0.0019 0.0019 0.0012 0.0009 AF 0.0019 0.01 3.5 0.60 0.90 0.0018 0.0021 0.0017 0.0012 0.0013 AG 0.0023 0.007 3.5 0.60 0.90 0.0014 0.0021 0.0024 0.0012 0.0013 AH 0.0023 0.007 3.4 0.60 0.90 0.0018 0.0021 0.0024 0.0012 0.0013 BA 0.0022 0.01 3.6 0.40 0.60 0.0021 0.0018 0.0024 0.0013 0.0014 BB 0.0023 0.01 3.4 0.40 1.00 0.0016 0.0021 0.0024 0.0012 0.0013 BC 0.0021 0.01 3.5 0.60 0.70 0.0018 0.0024 0.0021 0.0012 0.0011 Steel Chemical Composition (mass %) symbol V Sn Sb Others Si—2Al—Mn Remarks AA 0.0012 0.03 0.001 Cu: 0.3 1.9 Invention steel AB 0.0009 0.04 0.001 Ni: 0.2 1.1 Invention steel AC 0.0012 0.04 0.001 Cr: 0.5 1.1 Invention steel AD 0.0012 0.04 0.001 Cu: 0.2, Cr: 0.3 1.1 Invention steel AE 0.0013 0.04 0.001 Cu: 0.2, Ni: 0.1 1.1 Invention steel AF 0.0011 0.04 0.001 Ni: 0.1, Cr: 0.4 1.1 Invention steel AG 0.0012 0.04 0.001 Cu: 0.1, Ni: 0.1, 1.1 Invention steel Cr: 0.2 AH 0.0012 0.04 0.001 1.0 Invention steel BA 0.0011 0.03 0.001 Cu: 0.02, Ni: 0.03 2.0 Invention steel BB 0.0009 0.03 0.001 Ni: 0.05, Cr: 0.05 1.0 Invention steel BC 0.0012 0.03 0.001 Cu: 0.5, Cr: 0.6 1.5 Comparative steel

TABLE 4-1 Rolling conditions in final cold rolling Average Total Reduction Friction Finish Steel Sheet rolling reduction rate R_(s) coefficient annealing sheet Steel thickness speed rate R_(t) in final in final temperature No. symbol (mm) (m/min) (%) pass (%) pass (—) R_(s)/R_(t) (° C.) 1 A 0.25 1000 87.5 25 0.03 0.29 790 2 A 0.25 80 87.5 25 0.03 0.29 790 3 A 0.25 500 87.5 50 0.03 0.57 790 4 A 0.25 500 87.5 10 0.03 0.11 780 5 A 0.25 500 87.5 24 0.20 0.27 780 6 A 0.25 600 87.5 25 0.03 0.29 740 7 A 0.15 550 92.5 35 0.03 0.38 760 8 A 0.30 650 85.0 21 0.03 0.25 760 9 A 0.35 700 82.5 20 0.03 0.24 720 10 B 0.20 500 90.0 32 0.03 0.36 750 11 B 0.25 600 87.5 28 0.03 0.32 750 12 B 0.25 600 87.5 14 0.03 0.16 770 13 B 0.25 600 87.5 38 0.03 0.43 750 14 C 0.25 400 87.5 25 0.03 0.29 740 15 D 0.20 400 90.0 28 0.03 0.31 780 16 F 0.25 700 87.5 24 0.03 0.27 790 17 H 0.20 650 90.0 26 0.03 0.29 730 18 I 0.20 500 90.0 26 0.03 0.29 750 19 J 0.20 500 90.0 26 0.03 0.29 750 20 K 0.20 500 90.0 26 0.03 0.29 790 21 M 0.20 500 90.0 26 0.03 0.29 790 22 N 0.20 500 90.0 26 0.03 0.29 790 23 O 0.25 700 87.5 28 0.03 0.32 770 Steel sheet properties after finish annealing Area Average ratio of Average magneto- Steel recrystal- grain size of Yield striction Iron loss sheet lization recrystallized stress value W_(10/400) No. (%) grains (μm) (MPa) λ_(p−p)(×10⁻⁶) 9 + 62*t (W/kg) Remarks 1 96 50 450 3.4 24.5 22.5 Comparative Example 2 35 25 530 8.4 24.5 34.9 Comparative Example 3 97 51 445 3.2 24.5 22.4 Comparative Example 4 38 21 535 8.9 24.5 35.9 Comparative Example 5 98 55 440 3.7 24.5 22.9 Comparative Example 6 70 20 520 2.5 24.5 19.8 Invention Example 7 75 26 528 2.6 18.3 16.2 Invention Example 8 75 26 528 2.7 27.6 20.4 Invention Example 9 68 21 560 2.4 30.7 21.7 Invention Example 10 72 25 570 2.9 21.4 18.4 Invention Example 11 72 25 570 2.8 24.5 19.9 Invention Example 12 80 24 560 2.8 24.5 20.3 Invention Example 13 72 28 565 2.8 24.5 19.8 Invention Example 14 65 20 420 3.9 24.5 22.9 Comparative Example 15 80 28 630 2.7 21.4 17.5 Invention Example 16 85 25 580 3.1 24.5 19.4 Invention Example 17 64 24 535 3.7 21.4 19.1 Invention Example 18 72 30 570 2.0 21.4 16.9 Invention Example 19 72 30 570 1.9 21.4 16.7 Invention Example 20 85 35 555 2.1 21.4 17.2 Invention Example 21 85 35 555 1.8 21.4 16.6 Invention Example 22 85 35 555 2.0 21.4 16.9 Invention Example 23 78 32 565 1.9 24.5 16.7 Invention Example *Steel sheets produced from steels E, G, L, and Q are not described because sheet breakage has been caused during cold rolling.

TABLE 4-2 Rolling conditions in final cold rolling Average Total Reduction Friction Finish Steel Sheet rolling reduction rate R_(s) coefficient annealing sheet Steel thickness speed rate R_(t) in final in final temperature No. symbol t (mm) (m/min) (%) pass (%) pass (—) R_(s)/R_(t) (° C.) 24 P 0.25 700 87.5 28 0.03 0.32 770 25 R 0.30 750 85.0 22 0.03 0.26 750 26 S 0.30 700 85.0 22 0.03 0.26 750 27 T 0.30 700 85.0 22 0.03 0.26 750 28 U 0.30 700 85.0 22 0.03 0.26 750 29 V 0.30 700 85.0 22 0.03 0.26 750 30 W 0.25 600 87.5 25 0.03 0.29 760 31 X 0.25 600 87.5 25 0.03 0.29 760 32 Y 0.25 600 87.5 25 0.03 0.29 760 33 Z 0.25 600 87.5 25 0.03 0.29 760 34 Z 0.30 650 85.0 25 0.03 0.29 850 35 Z 0.30 650 85.0 25 0.03 0.29 650 36 AA 0.25 600 87.5 24 0.025 0.27 710 37 AB 0.25 600 87.5 24 0.025 0.27 710 38 AC 0.25 600 87.5 24 0.025 0.27 710 39 AD 0.25 600 87.5 24 0.025 0.27 710 40 AE 0.25 600 87.5 24 0.025 0.27 710 41 AF 0.25 600 87.5 24 0.025 0.27 710 42 AG 0.25 600 87.5 24 0.025 0.27 710 43 AH 0.25 600 87.5 24 0.025 0.27 710 44 BA 0.25 500 87.5 22 0.025 0.25 780 45 BB 0.25 500 87.5 22 0.025 0.25 780 46 BC 0.25 500 87.5 22 0.025 0.25 780 Steel sheet properties after finish annealing Area Average ratio of Average magneto- Steel recrystal- grain size of Yield striction Iron loss sheet lization recrystallized stress value W_(10/400) No. (%) grains (μm) (MPa) λ_(p−p)(×10⁻⁶) 9 + 62*t (W/kg) Remarks 24 78 32 565 2.0 24.5 16.9 Invention Example 25 72 30 510 7.2 27.6 32.4 Comparative Example 26 72 30 510 7.6 27.6 32.8 Comparative Example 27 72 30 510 7.9 27.6 33.6 Comparative Example 28 72 30 510 7.4 27.6 32.6 Comparative Example 29 72 30 510 7.2 27.6 32.4 Comparative Example 30 75 32 560 2.9 24.5 19.7 Invention Example 31 75 32 540 2.7 24.5 19.5 Invention Example 32 75 32 570 2.8 24.5 19.6 Invention Example 33 75 32 555 2.7 24.5 19.5 Invention Example 34 100 60 465 3.5 27.6 25.3 Comparative Example 35 10 15 550 7.2 27.6 38.9 Comparative Example 36 50 22 540 2.8 24.5 16.8 Invention Example 37 50 22 570 2.7 24.5 16.3 Invention Example 38 50 22 570 2.7 24.5 16.2 Invention Example 39 50 22 570 2.7 24.5 16.1 Invention Example 40 50 22 570 2.7 24.5 16.1 Invention Example 41 50 22 570 2.7 24.5 16.0 Invention Example 42 50 22 570 2.7 24.5 15.9 Invention Example 43 50 22 550 2.8 24.5 16.9 Invention Example 44 80 29 555 2.6 24.5 17.1 Invention Example 45 80 29 555 2.9 24.5 17.2 Invention Example 46 80 29 555 2.7 24.5 27.6 Comparative Example *Steel sheets produced from steels E, G, L, and Q are not described because sheet breakage has been caused during cold rolling.

Furthermore, the obtained steel sheets Nos. 6 to 11 are subjected to a stress relief annealing of 825° C.×1 hr (N₂ atmosphere) as in the stress relief annealing of the stator core, and thereafter the iron loss in the L+C directions is measured by an Epstein test and also an average crystal grain size observed at a section in the rolling direction is measured. The results are shown in Table 5. As seen from these results, both high strength and low iron loss can be achieved in the material according to the invention.

TABLE 5 Steel sheet properties after stress relief annealing Steel Average grain size Iron loss sheet Steel of recrystallized W_(10/400) No. symbol grains (μm) (W/kg) Remarks 6 A 120 10.5 Invention Example 7 A 130 8.5 Invention Example 8 A 130 11.1 Invention Example 9 A 110 11.9 Invention Example 10 B 115 9.2 Invention Example 11 B 115 10.4 Invention Example 

The invention claimed is:
 1. A non-oriented electrical steel sheet having a chemical composition comprising C: not more than 0.005 mass %, Si: 2.8 to 6.5 mass %, Mn: 0.05 to 2.0 mass %, Al: not more than 3.0 mass %, P: not more than 0.20 mass %, S: not more than 0.005 mass %, N: not more than 0.005 mass %, Ti: not more than 0.003 mass %, V: not more than 0.005 mass %, and Nb: not more than 0.005 mass %, provided that Si, Mn and Al satisfy Si—2Al—Mn≥0, and the remainder being Fe and inevitable impurities, wherein an average value of magnetostriction λ_(p-p) (L) in a rolling direction and magnetostriction λ_(p-p) (C) in a direction perpendicular to the rolling direction at a frequency of 400 Hz and a magnetic flux density of 1.0 T is not more than 4.5×10⁻⁶ and an area ratio of recrystallized crystal grains at a section in the rolling direction of the steel sheet is 40 to 95% and an average grain size is 10 to 40 μm.
 2. The non-oriented electrical steel sheet according to claim 1, which contains at least one group selected from the following groups A to D in addition to the above chemical composition: Group A: 0.0020 to 0.10 mass % in total of one or two selected from Mo and W; Group B: 0.005 to 0.20 mass % of one or two selected from Sn and Sb; Group C: 0.001 to 0.010 mass % in total of one or two selected from Ca and Mg; Group D: 0.01 to 1.0 mass % in total of one or more selected from Cu, Ni and Cr.
 3. The non-oriented electrical steel sheet according to claim 1, wherein a yield stress is not less than 500 MPa and a sheet thickness t mm and an iron loss W_(10/400) W/kg satisfy the following equation (1): W _(10/400)≤9+62×t . . . .  (1) where t is in the range of 0.1 to 0.35 mm.
 4. The non-oriented electrical steel sheet according to claim 2, wherein a yield stress is not less than 500 MPa and a sheet thickness t mm and an iron loss W_(10/400) W/kg satisfy the following equation (1): W _(10/400)≤9+62×t . . . .  (1) where t is in the range of 0.1 to 0.35 mm.
 5. A method of producing the non-oriented electrical steel sheet according to claim 1, the method comprising: hot rolling a steel slab having a chemical composition comprising C: not more than 0.005 mass %, Si: 2.8 to 6.5 mass %, Mn: 0.05 to 2.0 mass %, Al: not more than 3.0 mass %, P: not more than 0.20 mass %, S: not more than 0.005 mass %, N: not more than 0.005 mass %, Ti: not more than 0.003 mass %, V: not more than 0.005 mass %, and Nb: not more than 0.005 mass %, provided that Si, Mn and Al satisfy Si—2Al—Mn>0, and the remainder being Fe and inevitable impurities to form a hot-rolled sheet, subjecting the hot-rolled sheet to a hot-band annealing and then to a single cold rolling or two or more cold rollings having an intermediate annealing interposed therebetween to form a cold-rolled sheet having a final sheet thickness, and subjecting the cold-rolled sheet to a finish annealing, wherein an average rolling speed in a final cold rolling is 100 to 900 m/min; a friction coefficient in a final pass is 0.01 to 0.10; a ratio (R_(s)/R_(t)) of rolling reduction R_(s) in the final pass (%) to a total rolling reduction R_(t) (%) is 0.15 to 0.45; and a soaking temperature in the finish annealing falls within the range of 700 to 820° C.
 6. The method for producing a non-oriented electrical steel sheet according to claim 4, wherein the steel slab contains at least one group selected from the following groups A to D in addition to the above chemical composition: Group A: 0.0020 to 0.10 mass % in total of one or two selected from Mo and W; Group B: 0.005 to 0.20 mass % of one or two selected from Sn and Sb; Group C: 0.001 to 0.010 mass % in total of one or two selected from Ca and Mg; Group D: 0.01 to 1.0 mass % in total of one or more selected from Cu, Ni and Cr.
 7. A method for producing a motor core comprising a stator core and a rotor core, wherein an average crystal grain size of the rotor core is 10 to 40 μm, the method comprising: laminating the non-oriented electrical steel sheet according to claim 1 having an average crystal grain size of 10 to 40 μm to form the stator core, subjecting the stator core to a stress relief annealing thereby to have an average crystal grain size of the stator core being not less than 80 μm.
 8. A method for producing a motor core comprising a stator core and a rotor core, wherein an average crystal grain size of the rotor core is 10 to 40 μm, the method comprising: laminating the non-oriented electrical steel sheet according to claim 2 having an average crystal grain size of 10 to 40 μm to form the stator core, subjecting the stator core to a stress relief annealing thereby to have an average crystal grain size of the stator core being not less than 80 μm.
 9. A method for producing a motor core comprising a stator core and a rotor core, wherein an average crystal grain size of the rotor core is 10 to 40 μm, the method comprising: laminating the non-oriented electrical steel sheet according to claim 3 having an average crystal grain size of 10 to 40 μm to form the stator core, subjecting the stator core to a stress relief annealing thereby to have an average crystal grain size of the stator core being not less than 80 μm.
 10. A method for producing a motor core comprising a stator core and a rotor core, wherein an average crystal grain size of the rotor core is 10 to 40 μm, the method comprising: laminating the non-oriented electrical steel sheet according to claim 4 having an average crystal grain size of 10 to 40 μm to form the stator core, subjecting the stator core to a stress relief annealing thereby to have an average crystal grain size of the stator core being not less than 80 μm. 